Because of their unique mechanical and electric properties, the recently discovered fullerene nanotubes, both Single-Walled Nanotubes (SWNTs) and Multi-Walled Nanotubes (MWNTs) have been investigated for many applications. Indeed these are materials of such widespread interest that the application development has out-paced its mass availability. The most added-value applications that are being developed using nanotubes include Field Emission Devices, Memory devices (high-density memory arrays, memory logic switching arrays), Nano-MEMs, AFM imaging probes, distributed diagnostics sensors, and strain sensors. Other key applications include: thermal control materials, super high tensile strength (about 20 times higher than steel) and light weight (one-sixth of steel) reinforcement and nanocomposites, EMI shielding materials, catalysts and catalytic support, gas storage materials, high surface area electrodes, and light weight and high conductance wires.
The as-produced by most of known techniques, carbon nanotubes are, in essence, large fullerenes, see B. I. Yakobson and R. E. Smalley, American Scientist, 85, 324-337 (1997). They are ideally composed of one or more closed-cap seamless cylinders made of rolled-up graphene network, known for its supreme in-plane strength, that imparts a carbon nanotube with exceptionally high longitudinal stiffness. Carbon nanotubes are predicted to have very high stiffness and axial strength, as a result of perfect structure and of their very high aspect ratios (L/D) compared to commonly used high strength fiber [Rao A M, Richter E, Bandow S, Chase B, Eklund P C, Williams K A, Fang S, Subbaswamy K R, Menon M, Thess A, Smalley R E, Dresselhaus G, Dresselhaus M S, Science 1997, 275, 187-191]. MWNT are typically 3-100 nm in diameter and with aspect ratio typically less than 100. The MWNT consists of 5 concentric graphene layers up to about 50 layers. However, the numbers of layers are very difficult to control. SWNT on the other hand consists of single graphene layer with diameter typically in the range of 1.2-3 nm and with very large (104-105) aspect ratio. The values of the Young's modulus for SWNTs and MWNTs were identified experimentally to be in the Tera-Pascal range, thus much exceeding that value for any other reinforcement materials, including the record number of about 800 GPa for famous carbon whiskers, first made by Roger Bacon, “Growth, Structure and Properties of Graphite Whiskers”, Journal of Applied Physics, 31, 283 (1960).
The electronic properties and applications of nanotubes are not less attractive. The SWNTs have large specific surface area, which has enhanced chemical reactivity due to its curvature, with the unique porous structure. Combined with excellent electrical conductivity, this peculiar feature makes nanotubes an ideal electrode material for advanced batteries and fuel cells, in particular, for lithium-ion battery electrodes. The inner hollow cavity of the tubes can incorporate foreign atoms; one of the appealing applications of this property is designing of metallic quantum nano-wires.
The field-emitting properties of carbon nanotubes are remarkable for several reasons. In particular, very large stable currents are produced from nanotube arrays using only moderate electric fields. However, several field-emitting characteristics need to be optimized for the application of nanotubes in flat-panel displays, which does appear to be on track presently in many research and engineering groups, as it is the most promising among other applications for shortly large-scale industrial implementation. One is the “turn-on” field, which is required to produce a current density of 10 μA/cm2; the other is the “threshold” field that provides 1 mA/cm2 current density, which value is considered to be the minimum current density for applications in flat-panel displays. The lower the threshold field, the more suitable the emitter becomes for practical purposes. At the moment the SWNTs are the best performers for these parameters. However, any material for practical emitters must furnish acceptable performance durability. The MWNTs are very robust in low vacuum and thus meet this requirement, while SWNTs fail to perform the same perfectly, because of lower robustness to degradation at high currents. This implies, that some kind of thin multi-walled tubes, that would be of close to that of SWNTs diameter and yet retain the robustness of thick MWNTs, is the promising object for matching to optimal field emission parameters. Further, to obtain stable emission from the hypothetical thin MWNTs, they should be of uniform diameter and properties. The present invention has been made in the course of work aimed the experimental verification of these predictions. With the double-walled nanotubes (DWNTs) the anticipated advantages of thin-walled tubes for field emission came completely true, which is one of the objects of the present invention. The very study of DWNT properties, observation of field emission and discovery of its outstanding parameters have become possible upon obtaining the bulk quantities of almost pure DWNTs we have accomplished and described for the first time nationally and internationally in the present invention. We have shown that superior parameters of emitters, described in the present invention, are due to high purity of DWNTs employed in their manufacturing and their unique electronic properties. Before the present invention, no methods for selective production of thin-walled carbon nanotubes, including DWNTs, have been reported. Even those thin-walled nanotubes, including DWNTs, in principal exist, has been known from electron microscopy (EM) identification of very sparse such tubes in the carbon products formed in some high-temperature processes, mainly in the syntheses of SWNTs and/or MWNTs. No other means for their identification or property studies was ever employed, as DWNTs have not been available as a matter substance, even in microgram scale, nor could any their amount be separated from the mixture of said carbon products for positive identification. To obtain bulk amounts of almost pure DWNTs, we surprisingly found that by conducting experiments in conditions very close to those in [Liu C, Cong H. T, Li F, Tan P. H, Cheng H. M, Lu R, Zhou B. L. Carbon 1999; 37: 1865-1868], except that the normal method of electrode construction and orientation (i.e. ‘end to end’) were used, the macroscopic quantities of Double Walled Nanotubes (DWNTs) were formed as the dominant component in the product mixture, as we describe herein.
SWNTs have been discovered in an arc-discharge (AD) process of simultaneous vaporization of graphite and a metal catalyst, see S. Iijima and T. Ichihashi, “Single-Shell Carbon Nanotubes of 1-nm Diameter,” Nature 363, 603-605 (1993) and D. S. Bethune et al., Nature, 363, 605-607 (1993). The product contains SWNTs, fullerenes, amorphous carbon, graphite microparticles, naked and carbon coated metal particles. The catalyst and arcing conditions have been modified repeatedly in studies aimed to increase the SWNT yield, see, for EXAMPLE, C. Journet and P. Bernier, “Production of carbon nanotubes”, Appl. Phys. A67, 1(1998).
The AD technique is inexpensive and easy to implement, but initially it has been producing well below a currently common level of 20 wt. % of SWNTs on the average in the total soot produced. It was a development by Journet et al., “Large-Scale Production of Single-Walled Carbon Nanotubes by the Electric-Arc Technique,” Nature 388, 756-758 (1997), using a mixture of 4.2 at. % Ni and 1 at. % Y as a catalyst, that pushed the yield of the AD method to about 10-20% on average, with a small portion of the soot (the collaret) containing essentially larger amount of SWNTs, reaching 70%. The same high value was recently reported for the SWNT yield in a AD system, employing a sulfur-promoted Fe/Co/Ni catalyst and hydrogen-containing gas media, see C. Liu et al., “Semi-Continuous Synthesis of Single-Walled Carbon Nanotubes by a Hydrogen Arc Discharge Method”, Carbon, 37, 1865-68(1999). In this work, the geometry of the arc experiment was essentially different from the conventional one. Instead of a composite M/C anode rod, a wide shallow groove filled with a mixture of graphite and metal powders was used for an anode. The pencil-like sharpened cathode rod was tilted with respect to the working surface of the anode, instead of the ‘end to end’ anode-cathode geometry normally employed. These unusual conditions are considered to be important for obtaining SWNTs in a high yield. As assessed from electron micrographs value for the yield was 70 vol. %. Three well-resolved peaks at 1593 cm−1, 1576 cm−1 and 1569 cm−1 in the resonant Raman spectrum conform with abound presence of SWNTs in the as-produced material. The average diameter of the SWNTs, as determined from the high-resolution transmission electron microscopy (HRTEM) images, was 1.85 nm. That is, the SWNTs were generally thicker than those usually obtained in other known AD systems, where the diameter distribution averages to ˜1.3 nm.
Soon after the discovery of the AD method, a pulsed laser vaporization (PLV) of transition-metal/graphite composite pellets was found to produce high quality SWNT material, see P. Nikolaev et al., “Catalytic Growth of Single-Walled Nanotubes by Laser Vaporization,” Chem. Phys. Lett. 243, 49 (1995). Various modifications of the PLV technique have been made to improve the yield of SWNTs and to elucidate the mechanism of their formation, e.g., by using double laser pulses or by dividing the target into graphite and metal halves along the cylindrical axis, see A. Thess et al., “Crystalline Ropes of Metallic Carbon Nanotubes,” Science 273, 483 (1996) and M. Yudasaka et al., “Single-Wall Carbon Nanotube Formation by Laser Ablation using Double-Targets of Carbon and Metal,” Chem. Phys. Lett. 278, 102 (1997). Continuos laser and solar irradiation produce SWNTs as well, although with lower yield, see E. Munos et al., “Structures of Soot Generated by Laser Induced Pyrolysis of Metal-Graphite Composite Targets”, Carbon, 36, 525 (1998) and D. Laplaze et al., “Carbon Nanotubes: The solar Approach”, Carbon, 36, 685 (1998). The optimal metal concentration in the PLV pellets is 6-10 wt. %, the same as in AD anode rods. The pellet, vaporized by laser beam, is usually maintained at 1200° C., and Ar at 500 Torr is used as carrier gas. The SWNT produced with this method form bundles, that consist of about 100 SWNTs. The SWNT yield in the PLV method can be as high as 70-90 vol. %; however, the production rate is about an order of magnitude lower compared to conventional laboratory scale arc process. The bulk rate of SWNT-containing soot production can be substantially increased by rising laser power, however, at the expense of the reduction in SWNT yield, see, for EXAMPLE, A. G. Rinzler et al., “large-scale Purification of Single-Wall Carbon Nanotubes: Process, Product, and Characterization,” Appl. Phys. A 67, 29 (1998).
The catalytic chemical vapor deposition (CCVD) of carbon-containing gases for SWNTs synthesis has been first demonstrated with disproportionation of carbon monoxide at 1200° C. when molybdenum or Ni/Co particles supported on alumina are used as catalysts. The nanotube diameters in the range from 1 to 5 nm have been observed, and catalytic particles of the same size have been occasionally found attached to the nanotube end on HRTEM images. This result represents the first experimental evidence of SWNT production by the pre-formed catalytic particles. The CCVD of hydrocarbons for SWNTs was first reported by K. Hernadi et al., Carbon, 34, 1249-1257 (1996). Acetylene decomposition over silica or zeolite-supported transition-metal catalysts at 700° C. yields both SWNTs and MWNTs. The surface density and size of catalyst particles were found to be of importance in regulating the shape of the produced nanotubes. It was essential to highly disperse these metal catalysts on high surface area substrates to produce SWNTs. Large metal particles typically produce nanofibers. The SWNT production rate was below a gram per hour. The product properties varied greatly depending on the reagent gas used and the method of catalyst preparation.
A “floating catalyst” variant of the CCVD technique has been reported to produce SWNTs at 1200° C. from benzene or methane in the presence of metal iron catalyst, formed from the vapor of ferrocene drawn through the tubular reactor by hydrogen flow. The SWNTs thus obtained have larger diameters (˜1.75 nm) compared to those obtained by AD and PLV techniques (˜1.3 nm). The addition of thiophene was found to be effective in promoting the growth of SWNT and in increasing the yield of either SWNT or MWNTs under different growth conditions, see H. M. Cheng et al., “large-scale and low-cost Synthesis of Single-Walled Carbon Nanotubes by the Catalytic Pyrolysis of Hydrocarbons”, Phys. Lett. 72, 3282 (1998).
While we are in the search for efficient production of SWNTs in the arc discharge process and in CCVD systems for several years, and have attempted reproduction of some reported results. It was surprising to find that by conducting arc discharge experiments with Fe/Co/Ni/S catalyst in Hydrogen (H2)/Argon (Ar) atmosphere we have obtained the products, containing macroscopic quantities of double-walled nanotubes (DWNTs) along with concomitant minor quantities of SWNTs, contrary to what might be expected on the base of presented above results of Liu et al., Carbon, 37, 1865-68 (1999), that were obtained with similar catalyst and gas atmosphere. Upon optimization of the conditions for the high yield of DWNTs in the arc process, we became capable of producing DWNTs with high selectivity, corresponding to less than one (1) SWNT observed in HRTEM images per thirty (30) DWNTs, in accordance with the present invention.
Further, the arc process conditions for producing DWNTs (catalyst, gas atmosphere, chemical dynamics parameters) have been as closely as possible reproduced in attempts to obtain DWNTs by two variants of CCVD technique, including both thermal and high-frequency plasma assisted catalytic pyrolysis of hydrocarbons. These attempts eventually lead to efficient synthesis of DWNTs in both thermal and plasma-assisted variants, as described herein in accordance with the present invention.
DWNTs have already been observed as minor by-products in many catalytic systems for SWNT production, both in arc systems and in conventional CCVD systems employing the pyrolytic decomposition of hydrocarbons over metal catalysts, wherein the amount of DWNTs reported had never been in excess of a few percent that of SWNTs, see, for example, J. Kong et al, “Chemical Vapor Deposition of Methane for Single-Walled Carbon Nanotubes”, Chem, Phys. Letters, 292, 567 (1998); C. H. Kiang et al., “Catalytic Synthesis of Single-Layer Carbon Nanotubes with a Wide Range of Diameters”, J. Phys. Chem., 98, 6612 (1994); J. F. Colomer et al., “Large-Scale synthesis of Single-Wall Carbon Nanotubes by Catalytic Chemical Vapor Deposition (CCVD) Method”, Chem. Phys. Letters, 317, 83 (2000). Very rare DWNTs have been observed even in the products of pure graphite vaporization in the arc process, performed without intended addition of metals, see S. Iijima, Nature, 354, 56 (1991); T. W. Ebbesen, P. M. Ajayan, “Large-Scale Synthesis of Carbon Nanotubes”, Nature, 358, 220 (1992). Our invention presents the techniques for efficient and selective production of preponderant DWNTs, wherein they are the dominating product. It confers ready availability on the bulk quantities of DWNTs, thus rendering possible the pioneering studies of their partial physical and chemical properties, as well as exploration of these properties for various applications. Some of these properties of DWNTs have been assessed theoretically, and good prospects have been outlined for appropriate applications, see, for example, J. C. Charlier and J. P. Michenaud, “Energetics of Multilayered Carbon Tubules”, Phys. Rev. Letters, 70, 1858 (1993); D. H. Robertson et al., “Energetics of Nanoscale Graphitic Tubules”, Phys. Rev. B 45, 12592 (1992); J. Che et al., “Studies of Fullerenes and Carbon Nanotubes by an Extended Bond Order Potential”, Nanotechnology, 10, 263 (1999); S. M. Lee et al., “Hydrogen Adsorption and Storage in Carbon Nanotubes”, Synthetic Metals”, 113, 209 (2000); J. M. Bonard et al., “Field Emission From Single-Wall Carbon Nanotube Films”, Appl. Phys. Lett., 73, 918 (1998); O. Groening et al., “Field Emission Properties of Carbon Nanotubes”, J. Vac. Sci. Technol. B 18, 665 (2000). Generally, real and anticipated advantages of DWNTs over both MWNTs and SWNTs include higher yield in producing pure tubes, and lower production cost. In particular the advantages of DWNTs over MWNTs in structural applications include lower amount of defects, higher aspect ratio, higher strength due to smaller size, higher mechanical flexibility, lower density, and in electronic applications include better opportunities for controlling the tube electronic structure and properties, for example, by donor-acceptor doping or chemical functionalization. Advantages of DWNTs over SWNTs include more versatile electronic properties and greater opportunities for designing nano-scale electronic devices, theoretically predicted better performance in cold field emission, hydrogen storage, and lithium-ion batteries. The defectless structure of DWNTs may turn out to have the highest specific electric conductivity among the nanotubes, since in MWNTs, the electrical current flows only in a few outer layers of the tube, in agreement with magnetotransport experiments, see A. Bachtold et al., Nature, 397, 673 (1999). The DWNT could also be an ideal candidate for probe electrodes for scanning tunneling microscopy, as it is practically the same narrow and much more stiffer than a SWNT, while retaining the properties of SWNTs, such as flexibility, and the reversible buckling, rather than brittle breakage under the bending stress, which is characteristic of MWNTs. For the same reasons, the DWNTs are preferred over SWNTs and MWNTs for field emission and biological electrodes. We have found that electron field emission from DWNTs far surpasses in main parameters that of SWNTs, and described this experimental finding herein, in accordance with the present invention.
When a high electric field in the order of 107 V/cm is applied on a solid surface with negative electrical potential, electrons inside the solid is emitted into vacuum by the quantum mechanical tunneling effect. This phenomenon is called electron field emission. Such an extremely high field can be obtained on a sharp tip of a very thin needle, because electric fields concentrate at the sharp points. The carbon nanotubes possess the following properties favorable for field emitters: (1) favorable electronic structure, (2) good in plane electrical conductivity (3) a sharp tip, (4) high chemical stability, and (5) high mechanical strength. In 1995, field emission (FE) from an isolated single MWNT was first reported by Rinzler A G. Hafner J H. Nikolaev P, Lou L, Kim S G Tomanek D. Nordlander P. Colbert D T. Smalley R E. Science 1995:269:1550-3. Subsequently, many experimental results were published on FE for MWNTs such as by Collins P. G, Zettl A. Appl Phys Lett 1996:69:1969-70, Saito Y. Hamaguchi. K. Hata K. Uchida K, Tasaka Y. Ikazaki F, Yumura M. Kasuya A, Nishina Y. Nature 1997:389:554-5, and Bonard J. M, Maier F, Stoeckli T, Chatelain A. De Heer W A, Salvetat I. P, Forro L., Ultramicroscopy 1998:73:7-15, and for SWNTs such as by Saito Y, Hamaguchi K. Nishino T, Hata K. Tohji K. Kasuya, A, Nishina Y. Jpn J Appl Phys 1997:36:L1340-2, and Bonard J. M., Salvetat I. P., Stoeckli T., De Heer W. A., Forro L, Chatelain A. Appl Phys Lett 1998, 73:918-20. Very recently, nanotubes have been applied as cold electron sources in display devices by Saito Y., Uemura S., Hamaguchi K., Jpn J Appl Phys 1998, 37, L346-8 and successfully manufactured nanotube-based cathode-ray tube (CRT) lighting elements, which revealed stable electron emission, adequate luminance, and long life of the emitters. A recent study by Monteiro O. R, Mammana V. P, Salvadori M. C., Ager J. W, and Dimitrijevic S, Appl. Phys. A 71, 2000, 121-4, and it was shown that the turn-on field measured to be 2.3 V/μm and 2.6 V/μm for SWNTs and MWNTs, respectively.